In most hollow needle tufting machines, as typified by Kile, U.S. Pat. No. 4,549,496; Davis, et al., U.S. Pat. No. 5,588,383 and Ingram, U.S. Pat. No. 7,318,383, yarns are selectively fed to hollow needles by pneumatic pressure. Where the yarn being fed to a particular needle is changed, Kile and Davis found it necessary to retract the previously fed yarn from the hollow needle and to pneumatically urge the newly selected yarn to extend through the hollow needle to the appropriate length for tufting. Due to the characteristics of yarns and the imprecise nature of pneumatically supplied yarn, the lengths of yarns tufted are generally not uniform and the resulting fabrics not only require tip shearing but also result in the waste of substantial amounts of yarn.
Accordingly, the need exists to obtain more uniform stitch height with pneumatically fed yarns. Due to the elasticity of yarns, when tension is released from a yarn being fed for tufting, there is a contraction of yarn length. Different yarns have differing elasticities so the contraction is not precisely controllable. Furthermore, the amount of contraction varies with the length of yarn that has been placed under tension. Therefore, a need exists to provide for the provide for the feeding of yarns, and particularly the pneumatic feeding of yarns, in a fashion where only a relatively short length of yarn is placed under tension when the yarn is fed. In this fashion, the contraction of the yarn will be limited when the tension is released.
Additionally, even in the case of yarns fed by conventional means, varying yarn elasticity contributes to less uniform output. For instance, varying tension in pulling yarns from a yarn supply, and the release of tension after yarns are cut or otherwise released from a hook or looper, may cause different yarns to produce yarn bights of different heights.
Furthermore, some pneumatic yarn feeds are designed to constantly urge yarns to their associated hollow needles. In the absence of a tensioning device, the yarns will be fed at an incremental rate toward the hollow needle. Therefore, a need exists to prevent the slippage of yarns that are not selected for the current stitch.